Respiratory syncytial virus (RSV) infection is the most common cause of hospital admission for children, and in the elderly population RSV is the second most common cause of death from respiratory infection after influenza virus. Most studies of RSV infection on the cellular level have been performed in cultured, immortalized cells. In these cells, RSV binds to heparan sulfate (HS), a complex oligosaccharide on the cell surface, as its primary receptor to initiate infection. In primary, well differentiated human airway epithelial (HAE) cultures, an excellent model for the airway cells that RSV targets, RSV infects only ciliated cells and only via the apical surface. But this surface has no detectable HS. RSV must, therefore, use a different receptor to enter these cells. The goals of this project are to identify that receptor and the site on the RSV attachment protein that binds to it. Both will provide novel targets for the development of novel antiviral agents against RSV.